The present invention relates to an optical distance measuring apparatus for measuring a transit time of light between a time when the light is emitted and a time when the light reflected by an object to be measured is detected by a photodetector, to detect the distance to the object.
A so-called time-of-flight (TOF) method which is a method of measuring a time required for light to travel to an object and back to calculate the distance to the object has been widely known as a distance measuring method. This distance measuring method is a method of measuring a time Δt required for light to travel to an object and back and calculating the distance L to the object using the following Eq. (1) where the speed of light c is known as 3.0×1088 m/s.L=(c·Δt)/2  (1)
various specific signal processing methods in the TOF method have been proposed. For example, in a distance measuring apparatus disclosed in JP 6-18665A, a start pulse (synchronized with the operation of a light-emitting device) is used as a starting signal, electric charges are continuously stored in (or released from) an integrator until a stop pulse (received optical signal) is detected, and a round-trip time of light is obtained from the amount of increase (or decrease) in electric charge. Methods of measuring a time between the start pulse and the stop pulse include, for example, a method as used in the distance measuring apparatus disclosed in JP 7-294642A wherein counting the number of pulses of a reference clock is started simultaneously with the start pulse, and a round-trip time of light is obtained based on the number of pulses at a time when the stop pulse was detected.
However, in any of these methods, a current signal produced by a photodetector is converted to a pulse (voltage) signal, and signal processing is carried out with time information added to the pulse waveform. In general, an object to be measured is not specified, and the dynamic range of the amount of reflected light from an object to be measured or the like is very wide, so that in many cases there may be more noise components attributable to background light such as natural light than signal components. Under such a circumstance, it is very difficult to eliminate the noise attributable to the background light to extract the signal light pulse appropriately. Furthermore, a voltage waveform easily delays in phase under the influence of an environment (mainly a temperature environment), etc. For this reason, variation of a voltage waveform on the time axis becomes very large, and some time-correction means is thus required. In this case, the circuit configuration becomes very complicated, and will eventually lead to increase in manufacturing cost.
In contrast to this, R. Miyagawa et al. disclose that with using a photogate having a typical charge coupled device (CCD) structure, distance information can be obtained by processing a photoelectric current before converting a received optical signal to a voltage (“CCD-Based Range-Finding Sensor” IEEE Transactions on Electron Devices, Vol. 44, No. 10, October, 1997, pp. 1648-1652). FIG. 17 is a schematic cross-sectional view of an example of a photodetector having a photogate structure proposed by R. Miyagawa et al. FIG. 18 is a timing chart depicting the operation of the photogate structure.
In FIG. 17, the reference numeral 101 denotes a p-type semiconductor substrate, the reference numeral 102 denotes an n-type semiconductor layer forming a light-receiving section together with the p-type semiconductor substrate 101, the reference numeral 103 denotes an n-type semiconductor layer forming an electric charge storage section on a channel A (Ach), and the reference numeral 104 denotes an n-type semiconductor layer forming an electric charge storage section on a channel B (Bch). Furthermore, the reference numerals 105 and 106 each denote a gate having a metal oxide semiconductor (MOS) structure. The n-type semiconductor layer 102, the electric charge storage section 103, and the gate 105 constitute a switching MOS transistor 107 on Ach. Likewise, the n-type semiconductor 102, the electric charge storage section 104, and the gate 106 constitute a switching MOS transistor 108 on channel Bch.
A light-emitting device (not shown) applies light to an object to be measured according to the timing shown in FIG. 18(a). An optical signal reflected by the object is detected by the light-receiving section constituted by the p-type semiconductor substrate 101 and the n-type semiconductor layer 102 in FIG. 17, and becomes a received optical signal as shown in FIG. 18(b). As shown in FIGS. 18(a) and 18(b), the received optical signal is delayed from the emitted optical signal by a time (t1) required for the light to travel to the object and back. The gate 105 of the switching MOS transistor 107 on Ach is turned on and off in synchronization with the emitted optical signal, and the gate 106 of the switching MOS transistor 108 on channel Bch is turned on at a time when the gate 105 is turned off. In this case, the duration of the level “H” of the gate signals GA and GB which are input to the gates 105 and 106 is equal to the duration t0 of the level “H” of the emitted optical signal.
Switching operations of the switching MOS transistors 107 and 108 are carried out with the above timing, so that electric charges from the n-type semiconductor 102 for the period of time of (t0-t1) shown in FIG. 18(e) are stored in the electric charge storage section 103 on Ach, and electric charges from the n-type semiconductor 102 for the period of time of (t1) are stored in the electric charge storage section 104 on channel Bch. These operations are repeated to increase signal components (in other words, stored electric charges) stored in the electric charge storage section 103 and 104, and then the signals on both of the channels are read out. The distance to the object can be measured by, for example, calculating the ratio between both of the signals.
According to the photogate structure shown in FIG. 17, information about the amount of the phase delay corresponding to the round-trip time of light is processed as the amount of stored electric charges (intensity). For this reason, even if there is any temperature change or the like for example, it is not necessary to consider variations in phase at signal processing. Thus, steady distance measurement is possible.
Under a typical environment, there is some kind of background light such as sunlight or illumination light (light from a fluorescent lamp or the like). When there is background light, the background light is superimposed on the received optical signal wave shown in FIG. 18. The modulating frequency of background light varies between zero (DC light) (in the case of sunlight) and several tens of kHz (in the case of light from an inverter-controlled lamp), but is of the order of some kHz at most under a typical living environment. In contrast to this, the TOF method generally uses a high frequency of the order of several tens of MHz because of a delay time measuring method using the speed of light. For this reason, the frequency of background light is sufficiently low as compared with the pulse wave of the received optical signal, and may be assumed to be zero (DC light) within one cycle of the pulse wave. FIG. 19 is a timing chart in the case that there is background light. As shown in FIGS. 19(e) and 19(f), in the case that there is background light, the amount of electric charges stored in the electric charge storage section 103 on Ach increases by an amount of the background light received for the period of time when the gate 105 is on, and the amount of electric charges stored in the electric charge storage section 104 on channel Bch increases by an amount of the background light received for the period of time when the gate 106 is on. For this reason, the delay time t1 cannot be obtained using the amounts of electric charges stored in the electric charge storage section 103 on Ach and the electric charge storage section 104 on channel Bch.
As opposed to such a problem, in a distance image sensor disclosed in JP 2004-294420A, an additional electric charge storage section (not shown) different from the electric charge storage sections 103 and 104 is provided to a structure similar to that in FIG. 17, and the background light only is monitored in the third time period, whereby the reflected light only is extracted from the Ach output and Bch output. FIG. 20 is a timing chart for this distance image sensor. A switching MOS transistor (not shown) on Cch having a gate (not shown) which is turned on by a gate signal GC having the pulse width t0 subsequent to the gate signal GA on channel Ach and the gate signal GB on Bch as shown in FIG. 20 is provided around the light receiving section. In this case, there is no pulse signal based on the reflected light in the period of time when the gate signal GC is on, so that electric charges based on the background light only are stored and the intensity of the background light is monitored. Consequently, the distance to an object to be measured can be obtained from the three storage carriers (intensities) using the following Eq. (2) even under an environment where background light exists.
                                                                                          A                  -                  B                                                  A                  +                  B                  -                                      2                    ⁢                                                                                  ⁢                    C                                                              =                            ⁢                                                (                                                            t                      ⁢                                                                                          ⁢                      0                                        -                                          2                      ⁢                                                                                          ⁢                      t                      ⁢                                                                                          ⁢                      1                                                        )                                                  t                  ⁢                                                                          ⁢                  0                                                                                                                        →                                  t                  ⁢                                                                          ⁢                  1                                            =                            ⁢                                                                    t                    ⁢                                                                                  ⁢                    0                                    2                                ⁢                                  (                                      1                    -                                                                  A                        -                        B                                                                    A                        +                        B                        -                                                  2                          ⁢                                                                                                          ⁢                          C                                                                                                      )                                                                                        (        2        )            In Eq. (2), A is the amount of electric charges stored in the electric charge storage section 103 on a channel Ach, B is the amount of electric charges stored in the electric charge storage section 104 on Bch, and C is the amount of electric charges stored in the electric charge storage section (for background light) on Cch.
However, the distance image sensor disclosed in JP 2004-294420A has a problem as described below. That is, the brightness of background light as described above is up to several hundred thousands of luxes under sunlight outdoors, for example, and is several thousands of luxes even indoors such as in an office lighted relatively well. As is easily calculated, a photoelectric current obtained from such strong background light becomes the order of milliamperes or larger when an ordinary photodiode is used as the photodetector, although it depends on the optical system or the light-receiving area of the photodiode. In contrast to this, the amount of light reflected by an object to be measured is heavily dependent on the state of reflection at the surface of the object and the distance to the object, so that, for example, even if a high-power laser diode (LD) (several hundreds of milliwatts) is used as the light-emitting device, there is a case that when the distance to the object is of several meters, the amount of light incident to the photodetector becomes small to the order of nanowatts.
Under such an environment, the SN ratios of electric charges stored in the storage sections 103 and 104 in FIG. 17 are very low, and a trace amount of signal components thus exist among noise components which make up the major portion of the received optical signal. Since the capacities for electric charges of the storage sections 103 and 104 are limited, the number of repetitions of storage operations of electric charges is limited by the noise components, so that the lower the SN ratios, the larger the error of a measured distance becomes.